The basic steps of xerographic printing are well known. A charge retentive member, typically called a photoreceptor, is initially uniformly charged to a predetermined bias. In a subsequent exposure step, specific areas of the photoreceptor are selectably discharged according to an image desired to be printed. In it the case of an analog copier, focused light reflected from of the original image is used to expose the photoreceptor to copy the original image. In the case of a digital or laser printer, a modulating laser, scanning across a the moving photoreceptor, it is operated according to a digital image data, and on those small locations on the photoreceptor in which in the laser is on, the initial charge on the photoreceptor is discharged by the laser. Following of the exposure step, the imagewise-discharged photoreceptor it is developed by applying marking of particles, generally known as toner, to the photoreceptor. Typically, the of marking particles attached to the photoreceptor in that the charge areas thereof, which correspond to the print-black areas of the image it desired to be printed.
Following the developing step, the particles which are attached electrostatically to the photoreceptor are transferred, again by an electrostatic bias, into a print sheet, such as of paper. Finally, the paper with the transfer of marking particles there on a is fused, or heated, so that of the particles are permanently attached to the print sheet. (In the above discussion, the basic case of a monochrome xerographic printer or copier is described, but it will be apparent that the same principle will apply to the color xerography context as well.)
The present invention is directed to an improvement to the xerographic process, which addresses the problem of uneven wear experienced by a photoreceptor. Particularly in an office context, where numerous page images are output by a printer, all of the page images having a similar general set of margins, certain portions of the photoreceptor, particularly those corresponding to the side margins, will almost never be used for the printing of black portions of an image. In other words, because in that the office context the margin portions of an image will always be discharged by a laser or other exposure device, these portions of the photoreceptor will experience a different level of wear compared to those portions of the photoreceptor surface which are relatively less often discharged by the laser.
In certain contexts of xerographic printing, particularly in high volume situations where parts such as photoreceptors must be replaced on a regular basis, it is desirable to have the discharge of the entire surface of the photoreceptor evenly distributed, in order to optimize the use of the photoreceptor. The present invention is directed toward a method of operating a xerographic printer such that, in addition to the basic exposure step required for creation of the desired image on the photoreceptor, a supplementary exposure step is carried out, directed toward portions of the photoreceptor surface which were not exposed during of the standard exposure step.
Certain designs of photoreceptor are known to change their electrical characteristics depending on the total integrated exposure that any particular small area of the photoreceptor has received through its operational life. In other words, a portion of a photoreceptor surface of that is exposed fairly often will in effect age faster than another small area of the same photoreceptor which is exposed by the laser relatively less often. This unevenness in the aging of the photoreceptor surface can result in print quality defects, particularly in color printers, where a small changes in the development voltage associated with the photoreceptor and various stages in the printing process may result in an unacceptable variation in color reproduction.